She can't access her client's account with her own domain's IP. She must use the main server IP (if the account is an IP-less account) or the IP assigned to her client.

If she can't access ftp.clientdomain.com, then she needs to check to make sure clientdomain.com has an A record to main server's ip. That FTP CNAME seems fine to me as long as domainname.com = clientdomain.com, not resellerdomain.com